This invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery using carbonaceous material as the anode active material.
In recent years, with popularization of portable equipment such as a video camera or a radio cassette, etc., there has been an increased demand for a secondary battery which can be repeatedly used in place of a primary battery which is disposable.
Most of secondary batteries currently in use are a nickel-cadmium battery using an alkali electrolytic solution. However, since this nickel-cadmium battery has a low voltage of about 1.2 volts, it is difficult to improve the energy density. A further drawback is that sufficient life time cannot be obtained because the self-discharge rate is a higher value of more than 20% in a month at an ordinary temperature.
In view of this, there has been proposed a non-aqueous electrolyte secondary battery in which a non-aqueous solvent is used as an electrolytic solution and light metal such as lithium, etc. is used as the anode. Such non-aqueous electrolyte secondary battery advantageously has a high voltage of more than 3 volts, high energy density, and a low self-discharge rate. However, metal lithium, etc. used as the anode is gradually crystal-grown in the form of dendrite by repetition of a charge/discharge cycle, and comes into contact with the cathode. As a result, a short-circuit takes place in the battery. For this reason, such a non-aqueous electrolyte secondary battery has a short life time, so it is also difficult to put such a battery into practice.
In order to solve the problem of such an internal short-circuit, it is being studied to alloy lithium, etc. with other metal to use this alloy as the anode. In this case, however, since alloy is caused to be fine particles with repetition of the charge/discharge cycle, a sufficient life time cannot be obtained, resulting in difficulty in putting it into practice.
In view of this, there has been proposed a non-aqueous secondary battery as disclosed in the Japanese Patent Laid Open Application No. 90863/1987 publication in which carbonaceous material such as coke, etc. is used as the anode active material to carry out the charge/discharge cycle by doping/undoping of lithium. Such a secondary battery does not have the problems described above, and has an excellent cycle life time characteristic.
Further, when, e.g., Li.sub.x MO.sub.2 (M represents a single or more kinds of transition metals, and 0.05&lt;x&lt;1.10) as proposed by the applicant of this application is used as the cathode active material, the battery life time is further improved so that a non-aqueous electrolyte secondary battery having a desired high energy density can be provided. For this reason, great expectations are laid on this non-aqueous electrolyte secondary battery.
However, non-aqueous electrolyte secondary batteries using the carbonaceous material as the anode active material disadvantageously have a much higher self-discharge rate than that of batteries using metal lithium, etc. as the anode active material.
In order to improve such a drawback, e.g., in the Japanese Patent Laid Open Application No. 121248/1988 publication, it is disclosed that a carbonaceous material having grading distribution of more than 90% in terms of volume in the range from 0.1 to 50 .mu.m is used. However, even if carbonaceous material as described above is used, it is found that the self-discharge characteristic is not necessarily improved. Further improvement is necessary in order to obtain a non-aqueous electrolyte secondary battery having higher utility.